Field of the Invention
The present invention relates to a method for growing a silicon carbide crystal, and more specifically, a technique for obtaining a high quality single-crystal silicon carbide having few defects by suppressing not only a compositional change of a Si—C solution in a crucible but also precipitation of a polycrystal on the inner wall of a crucible and generation of a metal carbide, which is formed by binding an added metal element M and carbon C.
Description of the Related Art
Silicon carbide (SiC) is a material for a wide band-gap semiconductor and excellent in thermal conductivity and chemical stability, and has excellent basic physical characteristics as a power device also in view of transistor characteristics such as dielectric breakdown characteristics and a saturated drift rate. For these reasons, SiC is highly expected to be used as a material for a next generation power device and commercialization of a SiC power device has been reported.
Such a SiC substrate, however, has problems: the SiC substrate is expensive compared to a Si substrate. In addition, a high-quality single crystal SiC substrate having few defects is not sufficiently formed.
A main reason why it is difficult to produce a high-quality SiC single crystal substrate having few defects is that SiC does not melt under normal pressure. The melting point of Si, which is widely used as a substrate for a semiconductor device, under normal pressure is 1414° C. From a melt of Si, a high-quality single crystal large in diameter having few defects can be obtained by a CZ method or an FZ method.
In contrast, SiC, if it is heated under normal pressure, sublimates at a temperature of about 2000° C. Thus, crystal growth methods such as the CZ method and FZ method cannot be employed. Accordingly, at present, a SiC single crystal is mainly produced by a sublimation method including an improved Rayleigh method.
However, even if a power device is manufactured by using a SiC single crystal obtained by the sublimation method, characteristics of the power device might not be sufficient. This is because it is not easy to form a SiC single crystal having few defects. The phenomenon of crystal growth by the sublimation method is a precipitation directly from a gaseous phase. Because of this, the rate of crystal growth is low and it is difficult to control the temperature of a reaction space. As a result of recent improvements that research and development institutions have intensively made, the dislocation density of micro-pipes have decreased; however, many lattice defects, which have a significant effect upon electric characteristics of devices, such as threading screw dislocation, edge dislocation, basal plane dislocation, are still highly densely present.
Recently, a solution method for growing a silicon carbide crystal has been drawn attention (see, for example, Japanese Patent Laid-Open Nos. 2000-264790, 2004-002173 and 2006-143555). As described above, SiC itself does not melt under normal pressure. Then, in the solution method for producing a SiC single crystal, a Si melt is placed in a graphite crucible. In the Si melt, C is allowed to dissolve from a high-temperature region in a lower portion of the crucible to obtain a Si—C melt. A SiC seed crystal is brought into contact with the Si—C melt and SiC is epitaxially grown on the SiC seed crystal to obtain a SiC single crystal. In such a solution method, crystal growth of SiC proceeds in the state extremely close to thermal equilibrium. Thus, a SiC single crystal having few defects can be obtained compared to that obtained by the sublimation method.
As the solution method for obtaining a SiC single crystal, various processes are known. In “the most recent technology for a SiC power device” (in the first chapter, 1.2 Process for SiC solution growth, pages 41 to 43 (Science & Technology, published on May 14, 2010)), the processes are roughly divided into four categories: (a) Traveling Solvent Method (TSM), (b) Slow Cooling Technique (SCT), (c) Vapor Liquid Solid (VLS) Method and (d) Top Seeded Solution Growth (TSSG) Method. The term “solution method” used in the specification refers to Top Seeded Solution Growth (TSSG) Method.
In a solution method for producing a SiC single crystal, a Si melt is prepared and placed in a graphite crucible. Since the solubility of C in the Si melt is extremely low (about 1 at %), a transition metal and the like are generally added in the Si melt in order to facilitate dissolution of C (see, for example, Japanese Patent Laid-Open Nos. 2000-264790, 2004-002173 and 2006-143555).
The type and amount of such an additional element are determined in consideration of the following conditions: accelerating dissolution of C by the element; precipitating SiC as a primary crystal from the solution while the remainder is satisfactorily equilibrated as a liquid phase; precipitating none of a carbide and other phases by addition of the element; and stably precipitating a desired polymorph among the SiC crystal polymorphs, and obtaining a solution composition for increasing a single crystal growth rate as much as possible.
Conventional solution methods using a graphite crucible, however, have the following problems.
A first problem is that a solution composition gradually changes with the growth of a SiC single crystal, since a Si component runs out little by little from a Si—C solution. If the solution composition changes during growth of the SiC single crystal, the precipitation environment of SiC naturally changes. As a result, it becomes difficult to continuously grow the SiC single crystal, stably for a long time.
A second problem is a matter of an excessive dissolution of C derived from a graphite crucible. As a SiC single crystal grows, a Si component gradually runs out from the Si—C solution; whereas C is continuously supplied from the graphite crucible. Therefore, C is excessively dissolved into the Si—C solution, with the result that the Si/C component ratio in the solution changes.
A third problem is a matter of precipitation of a SiC polycrystal on the inner surface of the graphite crucible in contact with the Si—C solution. If C is excessively dissolved into the Si—C solution from the graphite crucible, as mentioned above, fine SiC polycrystals are likely to precipitate onto the inner surface of the graphite crucible. Such SiC polycrystals migrate through the SiC solution, reach near a solid-liquid interface between growing SiC single crystal and the Si—C solution, and inhibit growth of a single crystal.
The present invention was made in view of such problems of conventional methods. An object of the present invention is to provide a technique for obtaining a high-quality single crystal silicon carbide having few defects, compared to conventional methods using a graphite crucible, by suppressing not only a compositional change of a Si—C solution but also precipitation of a polycrystal on the inner wall of a crucible.